A microchip optomechanical accelerometer

21.06.2013, 00:00

Newsletter 6

Although the average person may not notice them, microchip accelerometers are quite common in our daily lives. They are used in vehicle airbag deployment systems, in navigation systems, and in conjunction with other types of sensors in cameras and cell phones. They have successfully moved into commercial use because they can be made very small and at low cost.  In general, accelerometers work by using a sensitive displacement detector to measure the motion of a flexibly mounted mass, called a proof mass.  Typically, detection of the motion of the proof mass is performed with an electrical read-out circuit.  Electrical circuits inherently suffer from thermal (Johnson) noise.  A simple laser and balanced pair of photodetectors, on the other hand, can straightforwardly realize shot-noise-limited detection over relevant bandwidth ranges.  In recent work we have demonstrated such a shot-noise-limited optomechanical accelerometer, formed from a silicon microchip and using a specially designed photonic crystal optical cavity (see figure).  The photonic crystal cavity used in our work consists of two silicon nitride nanobeams, situated like the two sides of a zipper, with one side attached to the proof mass. When the proof mass moves, it changes the gap between the two nanobeams, resulting in a change in the resonance frequency of the "zipper" cavity.  With such a device, displacements of a few femtometres (roughly the diameter of a proton) can be probed on a timescale of a second and for a laser power of only ~ 100 microWatts. Independent of how low-noise one makes the read-out circuit, an accelerometer’s resolution is ultimately limited by the thermal Brownian motion of its proof mass.  An interesting aspect of the zipper cavity sensor is that the probe laser light used to read-out the proof mass motion, also applies a force that tends to reduce the thermal motion of the proof mass. This cooling down to 3 Kelvin in the current devices dramatically increases the dynamic range of the sensor to over
40 dB. Due to the recent investment into silicon photonics by companies such as Intel and IBM, ultimately we envision these sort of optical accelerometers being integrated with lasers and detectors in a monolithic silicon platform. Beyond consumer electronics, such sensors might also find application in harsh, noisy environments where more conventional sensors fail, such as in oil and gas exploration.

Contact: oskar.painter(at)mpl.mpg(dot)de
Group: Painter Division
Reference: A. G. Krause et al., Nature Phot. 6, 768-772 (2012).